Split LayoutStore by responsibility

[robinebers]

TL;DR

Splits the 889-line LayoutStore into focused grouping, ordering, pinning, presentation, and model files without changing behavior. This PR is stacked on #926 so reviewers see only the responsibility split.

What was happening

• One observable store file mixed persisted layout mutation with dashboard grouping, Customize projections, pin policy, navigation presentation, and small view models.
• The file substantially exceeded the repository's ~500-line convention and made unrelated behavior harder to locate.
• These responsibilities already had clear seams but remained physically interleaved.

What this changes

• Keeps the observable store and persisted mutation core in LayoutStore.swift.
• Moves read-only dashboard and Customize projections into LayoutStore+Grouping.
• Moves ordering helpers, pin policy, and transient presentation state into focused extensions.
• Moves the small projection/notice value types into LayoutStoreModels.
• Keeps every resulting source file at or below 500 lines with direct extensions rather than a wrapper or delegation layer.

Heads-up

• Stacked on Preserve and normalize saved layout identifiers #926 (codex/normalize-layout-persistence); do not merge this before its base PR.
• Cross-file Swift extensions require a small set of read-only backing references and helpers to be module-internal. Provider expansion storage and persistence remain write-constrained behind the validated mutator; the executable target exposes no public API, and no caller contract changes.
• PR Remove unreachable UI scaffolding and test-only APIs #924 removes code this PR moves. Whichever lands second needs one rebase; keep Remove unreachable UI scaffolding and test-only APIs #924's removals (availableToAdd, the global pin count, and unused projection fields) in the new focused files. The resolved tree is verified.
• This is organization-only: no runtime behavior, persisted format, documentation, copy, or visual output changes.

Tests

• LayoutStore-focused suite: 69 tests passed.
• Full suite: 967 XCTest cases passed with 3 expected skips; 3 Swift Testing cases passed.
• script/build_and_run.sh verify completed a release build, signed app staging, launch, and process verification.
• git diff --check passed; moved method bodies match the Preserve and normalize saved layout identifiers #926 base apart from file separators.
• Clean combined simulation with the companion LayoutStore test split: 69 focused and 967 full XCTest cases passed, plus 3 Swift Testing cases.
• Combined with PRs Enforce warning-free Swift builds #912, Remove unreachable UI scaffolding and test-only APIs #924, Preserve and normalize saved layout identifiers #926, and Split LayoutStore tests by concern #932 after preserving the dead-code removals: 83 layout-filtered tests and 965 XCTest cases passed (3 expected skips), plus 3 Swift Testing cases.

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Note

Low Risk
Pure file split with moved code and internal visibility tweaks; tests and PR description assert no runtime or persistence changes.

Overview
Refactors LayoutStore into smaller files so persisted mutations stay in LayoutStore.swift and read-only projections, helpers, and UI-facing state move into dedicated extensions and LayoutStoreModels.swift.

New files carry the moved logic: LayoutStore+Grouping (dashboard/Customize projections, spend-capable providers, orderedProviders), LayoutStore+Ordering (metricOrder, drag-reorder and merge helpers), LayoutStore+Pins (pin caps, denial reasons, pinnedGroups), and LayoutStore+Presentation (popover navigation forwarding, transient notice surfaces, cancelDrag). ProviderGroup, ProviderMetrics, ProviderRow, and customize notice types now live in LayoutStoreModels.swift.

To let extensions share state without a wrapper layer, navigation, pinNotice/shareNotice/customizeNotice, registry, and isProviderEnabled are let and module-internal instead of private. setProviderExpanded is grouped with other persistence helpers near the bottom of the core file. No intended behavior, persistence format, or caller contract changes—organization only.

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[robinebers]

Superseded by #952. The replacement keeps only the mechanical LayoutStore responsibility split, is stacked on the compact dead-code PR #950 instead of the rejected layout-normalization stack, and changes no persistence or behavior. It produces exactly three files at 491/345/60 lines; the normalized implementation hash is unchanged, 73 focused tests and 960 XCTest cases plus 3 Swift Testing cases pass, the signed app launches, and the replacement has zero review threads with all available checks green. I am closing this harder-to-review stacked version; its branch and history remain available.